Electro-plating is a commonly used method for depositing metal and metal alloys onto semiconductor wafers. In a typical electro-plating process, the surface of a wafer is deposited with a blanket metal seed layer such as a copper seed layer. The surface of the wafer may have patterns, for example, trenches. In addition, the top surface of the wafer may also have a patterned mask layer to cover some portions of the metal seed layer, while the remaining portions of the metal seed layer are not covered. The metal is deposited on the portions of the metal seed layer that is not covered.
For performing the electro-plating, the wafer is mounted on a clamshell, which includes a plurality of electrical contacts in contact with the portions of the metal seed layer that are on the edge of the wafer. The wafer is placed into a plating solution. The metal seed layer is connected to a negative end of a DC power supply, so that the metal seed layer acts as the cathode. A metal plate, which provides the ions of the metal that is to be plated, acts as the anode, wherein the plating solution separates the anode from the cathode. When a voltage is applied between the cathode and the anode, the atoms in the metal plate are ionized and migrate into the plating solution. The ions are eventually deposited on the wafer.